Human serum albumin (HSA), the most abundant, versatile and complex protein in human blood, account for 50-60% of the total protein in plasma. See Biochem Pharmacol. 2005 Nov. 25; 70(11): 1673-84. In human body, HSA makes a large contribution to plasma colloid osmotic pressure, mediates coagulation, helps to maintain normal microvascular permeability, and serves as a carrier protein for fatty acids, cholochrome, amino acid, steroid hormone, metal ions, and drug molecules. In human serum, the normal albumin range is 34-54 g/L. See Nlm.nih.gov. Retrieved 2010 May 12. But many diseases, including liver complaint (cirrhosis especially), nephrotic syndrome, tumors, protein-losing enteropathy, empyrosis, malnutrition and so on, will cause a decline in the level of serum albumin, while a high-protein diet and chronic dehydration will raise the HSA levels. So it has been clinically used for disease diagnosis to quantitatively determine the albumin levels in human blood. It will be necessary to give patients HSA by injection, when the HSA level is much lower than the normal level. It has been confirmed by a large number of clinical practices that monitoring the HSA level will be reference for the early diagnosis of liver or kidney disease, and diseases after operation. More importantly, urinary microalbumin has been widely used as an important biomarker for patients with renal damage (like diabetes, hypertension and poststreptococcal acute glomerulonephritis), endothelial dysfunction, cardiac disease and venous thromboembolism. The normal range of the renal small albumin in human is shown in Table 1.
TABLE 1The diagnostic value of the microalbuminuriaGenderLower limitUpper limitUnit24 hours urine30300mg/24 hshort haul urine20200μg/minsingle-point urine30300mg/L or μg/gsingle-point urine/Female3.525 or 35mg/mmolUrinary C-peptide30400μg/mgCreatinine RatioMale2.5 or 3.525 or 35mg/mmol30300μg/mg
At present there are kinds of methods for HA quantification, including electrophoresis, immunoassay methods, as well as dye-binding procedures. Wherein the electrophoresis method is time-consuming with low specificity and it is easy to overestimate the HSA concentrations. See Clinical Chemistry: Theory, Analysis, Correlation, Mosby, 5th edition, 2009. While immunoassay is a specific method for HSA quantification, but it is high-cost, so it is less used in clinical application. Although the application of dye-binding method is relatively wide, its specificity is unable to be guarantee, for the reason that this method is based on the conjugation of dye molecule and parts of HSA or modifying the function of individual amino acids, but other tissue proteins in body also have the ability to bind these dyes. Bromcresol green (BCG) and bromcresol purple are widely used to quantitative detect albumin in clinic on account of simplicity, cheap and relatively higher specificity. But due to its low sensitivity (see Clinical Chemistry: Theory, Analysis, Correlation, Mosby, 5th edition, 2009.), hard to ensure the quantitative accuracy, overvalued (See J Clin Pathol. 2003, 56, 780.) or underestimated (See Clin Chim Acta. 1986, 155, 83.) often appear, and dye-binding-based assays can be interfered by the nonspecific interference from biological samples (See Clin Chem., 2009, 55, 583). Therefore, it is necessary to develop an easy-operating, highly specific, accurate faithful quantitative method for HSA quantification.
In recent years, domestic and overseas scholars found that HSA has catalytic properties and participates in catalyzing the hydrolysis of various molecules with ester bond in structure (Biochem Pharmacol 2010; 79 (5):784-91 & J Biol Chem 2008; 283 (33):22582-90), after a specific cleavage of the carboxylic ester bond, the phenylbenzoate group covalently bound to the residues of HSA accompanying with releasing the hydrolysate that contains free phenolic hydroxyl group. Guangbo Ge et al. also found that some small molecules with ester bond in structure can be catalyzed by HSA, for examples, Boc5, receptor agonist of GLP-1, can only be catalyzed by albumin in vivo, while other human esterase don't participate in the hydrolysis of this compound (Eur. J. Pharm. Sci. 2013; 48: 360-9). Recent domestic and overseas researches suggest that we can design and develop pseudo-esterase activity-based specific fluorescent probes for activity evaluation and quantification of HSA. The present invention provides the applications of a specific fluorescent probe to HSA, it is a dibenzoyl ester derivative, which replaced the C4 hydroxyl of N-n-butyl-4-hydroxy-1,8-naphthalimide, the fluorescence emission spectrum of the pseudo-esterase activity-based hydrolysis product is significantly different from that of the substrate molecule. This hydrolysis reaction has the characteristics of high selectivity, metabolic product easy to detect.